Recovery of magnesium from vapor mixtures



. g-v 7, 1945. D. s. CHISHOLM 2,381,403

RECOVERY OF MAGNESIUM FROM VAPOR MIXTURES Filed Jan. 29, 1942 1407/1 eJ/um INVENTUR. 0007/0: J. 'C/v/Irho/m MAGNESIUM MIXTURES mo s or mom VAPOR.

Douglas S. Chisholm, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mich., a corporation. 9! Michigan Application January 29,1942, Serial No. 428,754

"6 Claims. (01. 75-67) This invention relates to the recovery of metallic magnesium from vapor mixtures thereof with non-condensable gases.

An object of the invention is to provide an improved and highly efiective method of recovthat pressure but which in addition simulta.-'

fined zone and directing therethrough a rapidly moving continuous stream of liquid magnesium supplied at a condensing temperature,"the'- velocity of the stream being sufliciently high that substantially all the vapor mixture becomes entrapped therein and is exhausted from the zonetogether therewith. Because of the entrapping action of the fast-moving stream and the turbulent fiow of the liquid therein, the vapor mixture is brought rapidly into very intimate contact with the condensing medium and is quickly distributed throughout the latter; condensation of 1 still the liquid and thus to allow the gas to bubble out of suspension. The resulting gas-free mag- I nesium may then be returned to the initial step of the process, a portion being withdrawn from the zone in the form of one or more jets which are directed into the restricted outlet, the diameter oi the jet and especially the rate of flow o1 the magnesium therein being maintained sufflciently high that substantially all the cross-sectionalarea of the outlet is filled by the efliuent magnesium. In this way, the entrapping action of the moving condensing magnesium is utilized most effectively, the vapor mixture being exhausted from the zone and the magnesium condensed with great rapidity. This evacuating action of a moving jet of magnesium may advantageously be used to maintain a predetermined subatmospheric pressure in the closely confined contact zone and any other closed apparatus in open communication therewith merelyby insuring that the velocity of the magnesium in the jet is suflicient to exhaust the zone at a rate suflicient to maintain the desired pressure, according to the general principles of a siphon jet ejector. The precise jet diameter and jet velocity necessary may readily be calculated for any desired capacity condenser according to known methods. The minimum pressure which can be produced in this manner is, of course, the vapor pressure of the liquid magnesium forming the jet. In general, it is desirable to operate with a jet velocity somewhat greater than is just sufficient to insure maintenance of this minimum pressure, since under these conditions the entire system readily stabilizes itself at the minimum pressure; considerable variations in the rate of flow of vapor mixture to be condensed and of the magnesium in the jet can be tolerated'without appreciable 1 part of the magnesium in the vapor mixture is condensed instantly on contact with the jet or stream of liquid magnesium, and hence is converted to the liquid state before it ever leaves the contact zone, a small proportion of the magnesium vapor may persist in the bubbles of noncondensable gas entrapped in the liquid magnesium after it leaves the contact zone. In the invention, however, even this minute quantity of magnesium may be readily condensed, if desired, by sharply increasing the static pressure on the stream of magnesium immediately after it leaves the contact zone, as by suddenly decreasing the velocity thereof, e. g. by introducing the rapidly flowing magnesium jet into a conduit of increased cross-sectional area. This sud-' den decrease in velocity is, of course, accompanied by an equally rapidincrease in the static pressure on the liquid, with the result that any surviving magnesium vapors are almost instantly condensed, both as a result of the pressure surge itself. and also of the sudden great diminution in the volume of the bubbles of the entrapped gas. By utilizing this efiect, virtually complete 2 and very rapid recovery 01 magnesium is readily accomplished.

The temperature of the molten magnesium in the Jet entering the contact. zone may be maintained at any value well-below the boiling point of magnesium at the existing pressure, the temperature chosen ordinarily being that at which the vapor pressure of magnesium is approximately equal to the pressure it is desired to maintain in the zone, since in this way the system may be made largely self-stabilizing as to pressure, as already mentioned. Temperatures of 650 C. to 670 C. are ordinarily employed.

The process of the invention is particularly ad vantageous in condensing magnesium from vapor mixtures with small proportions of non-condensable gases chemically inert thereto, e; g. these mixtures formed in the purification of magnesium by distillation, the inert gas present being usually hydrogen, helium, or a hydrocarbon supplied as a carrier medium, or for other purposes.

The principle of the invention may also be used in condensin magnesium directly from vapor mixtures thereof with carbon monoxide, such as are generated in the production of metallic magnesium by heating a magnesia-containing raw material with carbon at elevated temperatures. As is well known, this condensation is rendered difficult by virtue of the fact that at condensing temperatures the magnesium tends to interact with the carbon monoxide, and thus to be lost. Despite this difficulty, however, condensation of magnesium from such mixture is entirely possible according to the process of the invention because of several important factors. Thus, the process may be operated at greatly reduced pressures, under which conditions the rate of interaction of magnesium and carbon monoxide is lowered. likewise, due to the extreme rapidity with which condensation occurs in a jet of moving magnesium, the time during which the magnesium is exposed to the carbon monoxide is very short. In addition, when the process is carried out so that the velocity of the magnesium jet is suddenly decreased after it leaves the contact zone, the pressure being thereby instantly increased,

- the volume of the bubbles of carbon monoxide entrapped in the moving magnesium is at once so greatly reduced that the surface area of contact of carbon monoxide and magnesium is comparatively small, whereby the rate of interaction is lowered. The net effect of all these factors operating in the same direction is to render possible, according to the invention, the direct condensation, from admixture with carbon monoxide, of magnesium vapor in liquid magnesium, without unduly great loss of the magnesium being condensed.

The invention may be further explained with reference to the accompanying drawing which i1- lustrates diagrammatically one form of apparatus for condensing magnesium according to the invention.

The apparatus shown is used as a combined condenser and evacuating means for a conventional internally heated closed still I, for purifying magnesium by distillation thereof from nonvolatile impurities. The magnesium'vapor liberated in the still, together with non-condensable gas, which may be present in the material being purified by distillation or may be added as a carrlel gas, passes from the still I through a vapor tube 2 maintaining the still in operative communication with the confined contact zone of a multi-nozzle siphon jet ejector 3. In the ejector,

the vapor mixture comes into contact with a number of rapidly moving Jets of molten magnesium supplied through nozzles 4 which direct the Jets into the throat 5 of a Venturi tube forming the outlet of the elector. The Venturi-throat of the ejector exhausts into a run-down pipe or barometric leg 8, of a height at least'equal to the barometric height or magnesium, i. e. about 22 feet. This pipe 8 terminates beneath the surface of a body of molten magnesium 1 which has already passed through the ejector and is maintained in a closed hot-well 8 provided at the top with a gas outlet 9. The hot-well 8 is divided into two portions by a dam I0, one portion serving to contain the molten magnesium 1, and the other portion acting as a reservoir I I for a second body of magnesium maintained at a lower level. Submerged within the magnesium in the reservoir II is a centrifugal pump I2 driven through a shaft I3 by variable speed means not shown and connected by a pipe I4 with the jet nozzles 4 within and freezingof molten magnesium by thermal insulation and heaters not illustrated.

In practice, the pump I2 is operated at a speed such that molten magnesium issues from the nozzles 4 at a velocity suflicient to fill the Venturi-throat 5 and to exhaust the still I at a rate sufilcient to maintain the latter at a. desired subatmospheric pressure. For instance, in apparatus in which the still I supplies magnesium vapor at a temperature of 660 C. and at a rate of 500 pounds per hour, and is to be maintained at a pressure of 4 millimeters of mercury absolute, the pump 12 should be adjusted to circulate magnesium at a rate of about 700 gallons per minute, the nozzles being of such diameter that the jet velocity is at least,40 feet per second, and the magnesium being maintained at a temperature of about 655 C. by cooling means, if necessary. Under these conditions the vapor mixture entering from the still I is entrapped in the jets of molten magnesium, the magnesium vapor being largely condensed on contact with the molten metal. Any surviving magnesium vapor entrapped in the jets is instantly condensed when the stream issues from the Venturi-throat 5 and is increased in cross-sectional area, whereupon the static pressure on the stream increases suddenly. The descending stream enters the pool I, where it is stilled and the entrapped non-condensable gas bubbles out of the molten metal, escaping through the outlet 9. The metal then overflows the dam I0 and is recirculated to the jet nozzles 4. A portion of the metal is drawn off continuously or at intervals through the outlet I6 as product.

The foregoing description is illustrative rather than limitative, the invention being co-extensive in scope with the following claims.

I claim:

1. In a method of recovering magnesium from a vapor mixture thereof with a non-condensable gas, the steps which comprise: admitting the vapor mixture to a confined zone and directing therethrough a rapidly moving continuous stream of liquid magnesium supplied at a condensing temperature, the velocity of the stream being sufficiently high that substantially all the vapor mixture becomes entrapped therein and is exnesium, the rate of flow of the latter being sumhausted from the zone together therewith, the magnesium vapor being simultaneously condensed; thereafter in .a second zone separating the entrapped, non-condensable gas from the effiuent liquid magnesium; and recovering the gasfree magnesium.

2. In a method of recovering magnesium from a vapor mixture thereof with a non-condensable gas,'--Ithe steps which comprise: admitting the vapor mixture to a closely confined zone maintained at sub-atmospheric pressure, said zone having an outlet of restricted cross-sectional area, and projet i g through the zone and into the outlet thereof at least one jet of molten magnesium, the rate of flow of the metal jet being sufliciently high that substantially all the cross-sectional area of the outlet is filled by the eflluent magneslum, and that the magnesium-containing vapor becomes entrapped in the molten magnesium and exhausted from the zone at a rate sufllcient to maintain the zone at the desired sub-atmospheric pressure, the magnesium vapor being simultaneously condensed through contact with the liquid magnesium; thereafter in a second zone separating the entrapped non-condensable gas gas, the steps which comprise: admitting the va- 30 por mixture to a closely confined zone maintained at sub-atmospheric pressure, said zone having an outlet of restricted cross-sectional area, and projecting through the zone and into the outlet thereof at least one jet of molten mag- 5 ciently high that substantially all the cross-sectional area of the outlet is filled by the eflluent magnesium and that the vapor mixture is entrapped in the jet and exhausted from the zone at a rate suflicient to maintain the zone at the desired sub-atmospheric pressure, the entrapped magnesium vapor being at least in part condensed in. the molten magnesium of the jet on contact therewith; and sharply increasing the static pressure on the eflluent magnesium immediately after it leaves the zone, whereby surviving entrapped magnesium vapor is instantly condensed; 'thereafter in a second zone separating the entrapped non-condensable gas from the eifluent liquid magnesium; and recovering the gas-free magnes'ium.

4. A method according to claim 3 wherein the vapor mixture is a mixture of magnesium-and a gas chemically inert thereto.

'5. A method according to claim 3 wherein the vapor mixture is a mixture of magnesium and carbon monoxide.

6. In an apparatus for condensing magnesium,

- in combination with a closed heating chamber for liberating magnesium vapor from a source material thereof: means for simultaneously exhausting the chamber and condensing-the magnesium vapor comprising a siphon jet ejector in operative communication with the chamber, a reservoir for liquid magnesium, and circulating means for withdrawing the magnesium from the source and supplying the same to the Jet ejector as the operating fluid thereof.

DOUGLAS S. CHISHOLM. 

